1. Field
Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to apparatus and methods of receive diversity (RXD) full cell search.
2. Background
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, currently supports various air interface standards, such as Wideband-Code Division Multiple Access (W-CDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). The UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSPA), which provides higher data transfer speeds and capacity to associated UMTS networks.
A wireless communication network may include a number of cells that can support communication for a number of user equipments (UEs). A UE may be within the coverage of one or more cells at any given moment. The UE may perform a search to detect cells and to acquire timing and other information for the detected cells.
During a cell search, a UE searches for a cell and determines slot synchronization, frame synchronization and code group identification, and a scrambling code of the cell. The cell search is typically carried out in three steps: Step 1: Slot synchronization, Step 2: Frame synchronization and code-group identification, and Step 3: Scrambling-code identification.
Step 1: Slot Synchronization
During the first step of the cell search procedure, a UE uses the Synchronization Channel (SCH) primary synchronization code to acquire slot synchronization for a cell. The UE may use a single matched filter (or any similar device) matched to the primary synchronization code, which is common to all cells. The slot timing of the cell may be obtained by the UE by detecting peaks in the matched filter output.
Step 2: Frame Synchronization and Code-Group Identification
During the second step of the cell search procedure, the UE uses the SCH secondary synchronization code to identify frame synchronization and a code group of the cell found in the first step. The UE may correlate the received signal with all possible secondary synchronization code sequences and then, based thereon, identify the maximum correlation value. Since the cyclic shifts of the secondary synchronization code sequences are unique, the specific code group of the cell, as well as the frame synchronization, may be determined by the UE.
Step 3: Scrambling-Code Identification
During the third and last step of the cell search procedure, the UE determines the exact primary scrambling code used by the cell identified in step 1. The primary scrambling code may be identified by the UE through chip-by-chip (or, alternatively, symbol-by-symbol) correlation over the Common Pilot Channel (CPICH) based on the codes within the code group identified in the second step. After the primary scrambling code has been identified, and based thereon, the UE may detect the Primary Common Control Physical Cannel (CCPCH). The UE then may read the system- and cell-specific Broadcast Channel (BCH) information from the Primary CCPCH.
If the UE has received information about which scrambling codes to search for, steps 2 and 3 above can be simplified. For example, a 2-step full search procedure may be used to provide an increase in efficiency over the above-noted 3-step full search.
The 2-step full search procedure may include Step A and Step B. In Step A, the UE performs a Step 1 (or step A) search to acquire the slot timing of neighboring cells. In Step B, for each multipath detected in Step A, the UE may determine the corresponding scrambling code and frame timing. A UE may determine a number of scrambling code hypotheses, which is equal to a number of neighbors N. Since a frame (which includes 38400 chips) consists of 15 slots (which includes 2560 chips), the UE may determine 15 hypotheses for frame timing. As such, the UE may correlate the received signal with 15N hypotheses for each multipath.
Thus, the aforementioned full search algorithms include two stages. The first stage (e.g., step 1 in the 3-step algorithm and step A in the 2-step algorithm) is to acquire slot timing and the second stage (e.g., steps 2 and 3 (or 2/3) in the 3-step algorithm and step B in the 2-step algorithm) aims to determine frame timing and scrambling code.
In a conventional system having receive diversity (RxD), e.g., a UE with a multiple antenna receiver, a full search algorithm may be described as follows. In each step of a full search, a searcher component (which may be, for example, part of a UE) combines the a ratio of pilot signal power (Ec) to total power (Io) received, or Ec/Io (which may also be referred to herein as “EcIo”) from two receive (Rx) antennas and detects cells when the sum Ec/Io exceeds a certain threshold. However, such an RxD full search implementation has several drawbacks.
First, the detection probability performance of the conventional RxD full search algorithm may degrade with an imbalance of Ec/Io. In field conditions, it is common to have receive (Rx) imbalance. For example, when a primary receive antenna (Rx0) is operating just above an automatic gain control (AGC) sensitivity level, the Primary Synchronization Channel (P-SCH) and CPICH Ec/Io from Rx0 may be much worse than P-SCH and CPICH Ec/Io from a secondary antenna (Rx1). In such scenarios, the conventional RxD full search may not be able to report peaks that could have been found in a non-RxD search. In addition, the conventional RxD full search algorithm only uses information related to the summed Ec/Io. Therefore, the conventional RxD full search algorithm may not detect an Ec/Io imbalance and, consequently, may not be able to determine the better antenna as between Rx0 and Rx1.
Second, the conventional RxD full search algorithm does not provide a scalable implementation, which may allow a UE to efficiently trade off detection probability and computational complexity. For example, step B in the 2-step full search can be very costly if the number of cells to search is large, especially for a UE that supports detected set cell search.
As such, improvements in receive diversity (RXD) full cell search are desired.